1. Field of the Invention
The present invention relates to a prism which may be incorporated, for example, in an optical head used for recording/reproducing signals on an optical disk, and a method for producing the prism. The present invention further relates to an optical beam shaping apparatus using the above-mentioned prism for shaping a spatial light intensity distribution (for example, for shaping from an oval distribution to a circular distribution) of a light beam such as laser light, and an optical head device employing such an optical beam shaping apparatus. The present invention further relates to a method for shaping a light beam.
2. Description of the Related Art
FIG. 1 is a schematic cross-sectional view showing an optical beam shaping apparatus disclosed in Japanese Laid-Open Publication No. 62-187321 as an example of a conventional optical beam shaping apparatus. Herein, a refractive index of air is referred to as n.sub.0 =1.
In the optical beam shaping apparatus shown in FIG. 1, a laser light 2 is emitted from a semiconductor laser 1, transmitted through a collimating lens 3, and is thereby converted into a parallel light 4. The parallel light 4, in turn, is incident on a surface 5A of a prism 5 made of a glass material (with a refractive index of n.sub.1) at an incident angle .psi..sub.1, (wherein .psi..sub.1 is an angle defined by the incident light 4 and a normal 5A' to the surface 5A of the prism 5). The incident light 4 is refracted at the surface 5A of the prism 5, and becomes a refracted light 4a having a refractive angle .psi..sub.1 ' with respect to the normal 5A' and an angle .theta..sub.2 (not shown) with respect to the incident light 4.
The refracted light 4a is then incident on a surface 5B (or "a bottom surface 5B" which opposes the surface 5A of the prism 5) at an incident angle .psi..sub.2 (wherein .psi..sub.2 is an angle defined by the light 4a and a normal 5B' to the surface 5B). The refracted light 4a reflects off the surface 5B and becomes a reflected light 4a'. An angle between the reflected light 4a' and the incident light 4 is referred to as an angle .theta..sub.2 ' (not shown).
The reflected light 4a' is incident on the surface 5A at an incident angle .psi..sub.21 (wherein .psi..sub.21 is an angle defined by the light 4a' and the normal 5A'), thereby being refracted by a refractive angle .psi..sub.21 ' to the normal 5A' and becomes emitting light 6. An angle between the refracted light 6 (i.e., the emitting light 6) and the original incident light 4 is referred to as an azimuth angle .theta..sub.21 ' (not shown).
When the surface 5A of the prism 5 is inclined by .alpha..sub.1 to the incident light 4, the incident angle .psi..sub.1 is characterized as follows: EQU .psi..sub.1 =.pi./2-.alpha..sub.1 Formula (1)
Further, the following Formula (2) is derived from Snell's Law at the surface 5A: EQU sin .psi..sub.1 =n.sub.1 sin .psi..sub.1 ' Formula (2)
Due to this refraction, the incident light 4 is either magnified or reduced by a factor of (cos.psi..sub.1 '/cos.psi..sub.1) within the refracting plane (i.e., within the plane of the drawing). The azimuth angle .theta..sub.2 of the refracted light 4a is given by the following Formula (3): EQU .theta..sub.2 =-.pi./2+.alpha..sub.1 +.psi..sub.1 ' Formula (3)
When the bottom surface 5B is inclined by .alpha..sub.2 to the incident light 4, the incident angle .psi..sub.2 is characterized as follows: EQU .psi..sub.2 =.pi./2-.alpha..sub.2 +.theta..sub.2 Formula (4)
Further, the following Formula (5) is derived from the Law of Reflection at the bottom surface 5B: EQU .theta..sub.2 '=.pi./2+.alpha..sub.2 -.psi..sub.2 Formula (5)
The incident angle .psi..sub.21 of the light 4a' to the surface 5A is given by the following Formula (6): EQU .psi..sub.21 =.pi./2+.alpha..sub.1 -.theta..sub.2 ' Formula (6)
Further, the following Formula (7) is derived from Snell's Law at the surface 5A: EQU n.sub.1 sin .psi..sub.21 =sin .psi..sub.21 ' Formula (7)
Due to this refraction, the light is further magnified or reduced by a factor of (cos .psi..sub.21 '/cos .psi..sub.21) within the refracting plane.
The azimuth angle .theta..sub.21 ' of the emitting light 6 is given by the following Formula (8): EQU .theta..sub.21 '=.pi./2+.alpha..sub.1 -.psi..sub.21 ' Formula (8)
Due to the two refractions at the surface 5A, the emitting light 6 is either magnified or reduced by a factor of m within the refracting plane, where m is given by the following Formula (9): EQU m=(cos .psi..sub.1 '/cos .psi..sub.1).multidot.(cos .psi..sub.21 '/cos .psi..sub.21) Formula (9)
By sequentially applying the above-mentioned Formulae (1) through (9), for example, when BK7 is selected as a glass material for forming the prism 5 under the following conditions: an oscillation wavelength of the semiconductor laser 1=0.64385 .mu.m (where n.sub.1 =1.51425); .alpha..sub.1 =17.59.degree.; and .alpha..sub.2 =31.340, an azimuth angle .theta..sub.21 ' of the emitting light 6 of 89.9963.degree. and a magnification ratio m of 2.501 are obtained. The traveling direction of the emitting light 6 is bent by an angle of about 90.degree. with respect to that of the incident light 4 and the beam is magnified about 2.5 times within the refracting plane.
In general, the parallel light 4 derived from the light 2 emitted from the semiconductor laser 1 has an oval spatial light intensity distribution (an oval cross-sectional intensity with an ellipticity of about 2.5). The above-described prism 5 magnifies the spatial light intensity distribution in a direction along a minor axis of the oval distribution, thereby obtaining the parallel light having a circular spatial light intensity distribution (a circular cross-sectional intensity).
However, such a conventional light beam shaping apparatus has the following problems.
A glass material forming the prism 5 always has a wavelength dependency of the refractive index (i.e., "dispersion"). Specifically, the refractive index of the light becomes smaller as the wavelength of the light becomes longer. For example, in the case where the prism 5 is made of BK7 under the conditions where an oscillation wavelength of the semiconductor laser 1 is 0.70652 .mu.m, the refractive index n.sub.1 of the prism 5 is 1.51243. Under this circumstance, the azimuth angle (emitting angle) .theta..sub.21 ' of the emitting light 6 is 89.9313.degree. which is smaller by 0.065.degree. than that in the above-described case where the oscillation wavelength of the semiconductor laser 1 is 0.64385 .mu.m.
Generally, due to variation in the output of the semiconductor laser 1, the oscillation wavelength is momentarily fluctuated several nanometers. When the oscillation wavelength is fluctuated, for example, by 10 nm in the above-described conventional optical beam shaping apparatus which employs the prism 5 made of BK7, the azimuth angle .theta..sub.21 ' of the emitting light 6 changes by 0.0104.degree..
In the case where the emitting light 6 is focused by an objective lens (e.g., with a focal length of 3 mm) so as to be used in an optical head for recording/reproducing signals in an optical disk, the above-mentioned change in the angle of 0.0104.degree. will result in a spot displacement of 0.54 .mu.m. This spot displacement of 0.54 .mu.m is not negligible when the optical head is used for reproducing signals recorded in signal pits of the optical disk on the order of submicrons, and may result in a fatal defect.